Within the field of semiconductor circuits, certain categories of circuitry require a reliable operation over a range of temperatures. One such circuit which may be used to provide a constant reference source is shown in FIG. 1. This simple circuit is a constant reference circuit that is implemented using a current source 101, producing an output current, which is proportional to absolute temperature (PTAT), coupled via a resistor 102 to a base-emitter voltage of a transistor 103, which is complementary to absolute temperature (CTAT). If the slope of the voltage drop across 102 (PTAT) balances the base-emitter voltage slope (CTAT), the output voltage provided at the common node of the current source 101 and the resistor 102 is substantially constant over temperature. Other known reference sources include those implemented using bandgap techniques.
A problem with providing such reference circuits is that there are a number of parameters which may provide variances in the performance of the reference circuit which leads to errors. For example, as a result of process variations in the transistors implemented in typical reference circuits, the base emitter voltage of the transistors may change. Even if the transistors are set to operate at the same emitter current, this may vary by as much as 10-20 mV, thereby causing errors in the reference output.
Another source of error is due to the typical technique used in the generation of the PTAT voltage in such reference circuits. The PTAT voltage is typically generated by means of a voltage AVBE, being the difference in the base-emitter voltages of two transistors provided in the circuit which operate at different current densities. The value of ΔVBE is then amplified to produce the required PTAT voltage value for use in such bandgap circuits. As the PTAT voltage is a scaled replica of ΔVBE, any offsets present in the ΔVBE are also amplified and added, which results in errors. Furthermore, additional errors may be introduced during the packaging of such circuits. For example, the moulding stress that may be applied to the transistor during the packaging may contribute to an additional shift in the reference value.
In typical bandgap voltage references, the output voltage requires trimming or adjusting so as to achieve a constant output voltage reference over a range of temperatures. This is typically achieved by altering the PTAT voltage, as due to the exponential relationship between the current and the base-emitter voltage of a bipolar transistor, it is more difficult to alter the CTAT voltage. Typically, both the absolute voltage and the temperature slope of a bandgap voltage reference must be trimmed, with the assumption that the base-emitter voltage of the bipolar transistor has a precise value at absolute zero. The base emitter voltage at zero Kelvin is known as the bandgap voltage. Due to the process variations, both the output voltage and the temperature slope or temperature coefficient (TC) for a real bandgap voltage reference will have different values from device to device. This causes problems if a precise absolute voltage and minimum temperature coefficient are required. When the PTAT voltage is adjusted at room temperature so as to correct the temperature slope, the adjustment turns the slope around 0 Kelvin, which causes the absolute voltage to also change. Therefore, once the temperature slope has been corrected, the absolute voltage must also be corrected. This correction in absolute voltage may in turn alter the temperature slope. As a result, the trimming process typically requires the step of correction of the temperature slope followed by the step of correction of the absolute voltage to be repeated several times. This means that when a precise absolute voltage and minimum temperature coefficient is required, a lengthy iterative process of trimming slope and absolute voltage must be employed.
Another way to trim the reference voltage is to record a minimum of two reference voltage values at two different temperatures, in order to find the temperature slope, and then to adjust the PTAT voltage by a corresponding amount and shift the reference voltage (or the gain) with a temperature constant value. However temperature trimming of units in production quantities using this technique has the drawback of requiring multiple handling and tracking of the individual units during temperature test.
A number of techniques have been developed to provide for the compensation of the temperature effect. An example of such a technique is disclosed in U.S. Pat. No. 6,075,354 (the content of which is incorporated herein by way of reference). In this document, three currents DAC's are provided to interface with a bandgap voltage generator, a first provided to trim first order temperature slope variations of the output reference voltage, a second to compensate for temperature slope curvature and a third to provide scalar gain adjustment. In order to adjust ΔVBE for the slope correction, the technique is used of pushing an external correction current through the first or second diode of the main bandgap cell. A drawback of this scheme is that as the PTAT voltage changes, the reference voltage slope also changes, which affects the absolute value of the reference voltage.
Another U.S. Pat. (No. 6,329,804 ) also describes a slope and level trim DAC for voltage references. In order to trim the reference voltage slope, a current switching DAC is used to inject a PTAT trimming current into one of the two diodes in the main bandgap cell. However, as in the case of U.S. Pat. No. 6,075,354, this patent also has the drawback that a change in ΔVBE changes both the slope and absolute value of the reference voltage.
There is therefore a need for a method and circuit that provides a simple way of trimming the reference voltage in which both reference voltage and temperature coefficient (or the slope) can be separately adjusted.